Would you literally type in 'localhost.localdomain, localhost'? Or would you substitute 'localdomain' with the actual domain?
– pgunstonDec 10 '14 at 1:39

@pgunston I believe you type "localhost" literally. The point being that "admin@localhost" would still get delivered to a local mailbox, but "admin@my.actual.domain.com" would get relayed off the machine.
– Darren CookDec 12 '16 at 10:25